Combustion engines such as diesel engines, gasoline engines, and gaseous fuel-powered engines are supplied with a mixture of air and fuel for combustion within the engine that generates a mechanical power output. In order to maximize the power output generated by this combustion process, the engine is often equipped with a divided exhaust manifold in fluid communication with a turbocharged air induction system.
The divided exhaust manifold increases engine power by helping to preserve exhaust pulse energy generated by the engine's combustion chambers. Preserving the exhaust pulse energy improves turbocharger operation, which results in a more efficient use of fuel. In addition, the turbocharged air induction system increases engine power by forcing more air into the combustion chambers than would otherwise be possible. This increased amount of air allows for enhanced fueling that further increases the power output generated by the engine.
In addition to the goal of maximizing engine power output and efficiency, it is desirable to simultaneously minimize exhaust emissions. That is, combustion engines exhaust a complex mixture of air pollutants as byproducts of the combustion process. And, due to increased attention on the environment, exhaust emission standards have become more stringent. The amount of pollutants emitted to the atmosphere from an engine can be regulated depending on the type of engine, size of engine, and/or class of engine.
One method that has been implemented by engine manufacturers to comply with the regulation of exhaust emissions includes utilizing an exhaust gas recirculating (EGR) system. EGR systems operate by recirculating a portion of the exhaust produced by the engine back to the intake of the engine to mix with fresh combustion air. The resulting mixture has a lower combustion temperature and, subsequently, produces a reduced amount of regulated pollutants.
EGR systems require a certain level of backpressure in the exhaust system to push a desired amount of exhaust back to the intake of the engine. And, the backpressure needed for adequate operation of the EGR system varies with engine load. Although effective, utilizing exhaust backpressure to drive EGR can adversely affect turbocharger operation, thereby reducing the air compressing capability of the air induction system. The reduced air compressing capability may, in turn, reduce the engine's fuel economy and possibly the amount of power generated by the engine. Thus, a system is required that provides sufficient exhaust backpressure to drive EGR flow without adversely affecting turbocharger or engine operation.
U.S. Patent Publication No. 2006/0174621 by Chen et al. published Aug. 10, 2006 (the '621 publication) discloses an internal combustion engine having a first turbocharger in fluid communication with a first exhaust manifold and fluidly communicating with an intake manifold, and a second similar turbocharger in fluid communication with a second exhaust manifold and fluidly communicating with the intake manifold in parallel with the first turbocharger. A crossover passage connects the first exhaust manifold to the second exhaust manifold, and a first exhaust gas control valve is located within the crossover passage to control communication between the first and second exhaust manifolds by way of the crossover passage. A second exhaust gas control valve is disposed between the second turbocharger and the second exhaust manifold at a location downstream of the crossover passage.
The engine described in the '621 publication is also provided with an EGR system including an EGR cooler connected in series with an EGR valve. A first EGR check valve and a second EGR check valve are disposed at a gas inlet side of the EGR cooler to allow exhaust from the first and second exhaust manifolds to recirculate back into the engine for subsequent combustion.
During operation of the engine described in the '621 publication, the first and second exhaust gas control valves are opened or closed based on operating conditions of the engine. By closing the first exhaust gas control valve and opening the second exhaust gas control valve, the first and second turbochargers operate similarly in parallel fashion to provide adequate boost and EGR back pressure at high engine speeds. By opening the first exhaust gas control valve and closing the second exhaust gas control valve, the second turbocharger is disabled at low engine speeds and exhaust from both the first and second exhaust manifolds passes through the first turbocharger by way of the crossover passage. This latter arrangement increases backpressure at low engine speeds such that EGR remains functional and adequate boost is provided.
Although the system of the '621 publication may adequately control exhaust gas recirculation in a turbocharged engine, it may be complicated and inefficient. Specifically, the system of the '621 publication requires complex valving and control mechanisms to change exhaust flow paths based on engine conditions. And, the system of the '621 patent requires the exhaust back pressure of all cylinders to be greater than the intake manifold pressure in order to achieve sufficient EGR flow. By applying backpressure to all cylinders, the engine efficiency may be compromised.
The disclosed exhaust system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.